Read cardano-node environment variables in hydra-node

[Noon]

The hydra-node binary, and the hydra-tui binary both depend on the node socket and --testnet-magic/mainnet configuration

It would be convenient if it used these from the environment by default, and then let them be overriden with command-line arguments.

I.e. we should at least look for these env vars:

CARDANO_NODE_NETWORK_ID
CARDANO_NODE_SOCKET_PATH